Such gearboxes are known, for example, from British Specification 2 074 669 and European Patent 0 224 407. They comprise an input shaft carrying fixed gear wheels, and first and second driven shafts, each shaft carrying loose gear wheels, all these shafts being rotatably mounted in a gearbox housing and each of the driven shafts engaging, via a respective fixed final drive pinion, with a driving gear of a differential, which drives front wheel drive shafts through constant velocity joints. Preferably all gears, including the reverse gear, are synchronized by couplers arranged adjacent to the loose gear wheels.
In the known multispeed change-speed gearboxes of the three-shaft type, an additional reverse gear shaft or axle is necessary in order to effect reversal of the direction of rotation for the reverse gear, so that the gearbox itself includes four shafts.
From German Patent 33 20 494 a multispeed change-speed gearbox for motor vehicles is known. That transmission is a gearbox of the two-shaft type, in which both the input shaft and the driven shaft carry fixed and loose gear wheels. The loose gear wheel of the reverse gear ratio is carried on a third shaft which, however, does not constitute a driven shaft.
In this known change-speed gearbox, reversal of the direction of rotation required for the reverse gear ratio is effected by meshing the loose gear wheel of the first gear, which is in mesh with the fixed gear wheel of the first gear, with the loose gear wheel of the reverse gear. In order to transmit the reversed driving torque of the reverse gear to the driven shaft, a further loose gear wheel for the reverse gear is required on the third shaft, and is meshed with a loose gear wheel of another gear on the input shaft. This latter loose gear wheel in turn meshes with its corresponding fixed gear wheel on the driven shaft, from which the driving torque is transmitted by way of the drive pinion to the differential.
The known change-speed gearbox has the disadvantage that providing two adjacent loose gear wheels required on the third shaft for the reverse gear increases the cost of manufacture and the space needed, and that the multiplicity of fixed and loose gear wheels of two forward gears that are involved in forming the reverse gear train have to be taken into account in designing the reverse gear. Thus, for example, the gear ratio of the reverse gear may be changed if, for technical reasons relating to the drive, it is necessary to change the gear ratio of one of the two forward gears involved in forming the reverse gear train.